If you add $multians->setCmpFlags(4, showTypeWarnings => 0); after $multians is defined, then it will let answers like "5" or "5,inf" into the checker. However, an answer that cannot be parsed into any MathObject like "(5,inf]" still won't get in. The only way to deal with answers like that is to use a post filter.
Thank you! I'm not sure I understand the post filter part, but will work on that. These two changes will help with many of the errors I had.